Known in the art is a submersible pump assembly comprising an electric motor having an electromechanical transducer shaft connected to a pump shaft, and a frequency converter electrically coupled to the electric motor (PCT, A1, WO 91/19905).
In this assembly, the converter is arranged in the area of the electric motor mount and is suitable for location with the submersible pump assembly directly in the well.
The problem with the pump assembly above is that a three-conductor cable should be used to supply power to the electric motor and the frequency converter. It makes the assembly less reliable and raises its operating cost, in particular, when the submersible pump assembly operates in the wells at large depths.
A well pumping system disclosed in Maximov V. P. "Adjustable Control of Submersible Pump System Drive", Moscow, 1981, pages 5,6, comprises a pump with an electric motor, each having its body suitable for location in a well using a tubing, and a frequency converter electrically coupled by a cable to the electric motor, the frequency converter including a rectifier and an inverter.
In this pump system, the frequency converter is located at the daylight surface in the vicinity of the well mouth and is connected by a multicore cable to the electric motor. In this case, voltage is also delivered to the electric motor by a three-conductor cable whose insulation has to meet stringent requirements. This raises the operating costs since the cable conductors are made of copper in order to provide the adequate electric connection, and failure of the insulation between the conductors may cause the risk of the system damage.